This article reports for the first time the use of a plastic optical fibre (POF) cantilever beam to measure the density of a liquid. The sensor is based on the Archimedes buoyancy principle. The sensor consists of a POF bonded on the surface of a metal beam in the form of a cantilever configuration, and at the free end of the beam a displacer is attached. Due to the apparent loss of the true weight of the displacer there is a deflection in the cantilever beam, which causes macro bending in the POF. The loss of intensity due to macro bending of the POF is a measure of the density of the liquid under test. The variation of weight loss with the density of different liquids showed that the weight loss is proportional to density. This sensor is capable of detecting the weight loss with respect to their densities even for liquids having close values of density like distilled water, tap water, and milk of various brands. The resolution of the sensor is observed to be 1.1 mg cm[superscript -3]. (Contains 6 figures and 1 table.)